The present invention relates to a two-speed transmission intended to be used on electric vehicles. The expression “electric vehicles” is used in the following description and claims to indicate all those vehicles which use an electric machine as prime mover, the electric machine being the only prime mover (it is the case of purely electric vehicles) or being associated to an internal combustion engine (it is the case of hybrid vehicles).
Transmissions having more than one speed, in particular two-speed transmissions, for use on electric vehicles are known and have the advantage, over single-speed transmissions, of allowing the electric machine to work at the optimal number of revolution per minute, that is to say, at the number of revolutions per minute corresponding to the maximum efficiency of the machine, over a wider speed range of the vehicle. This advantage applies both to the traction operating mode, in which the electric machine operates as a motor to generate the power required to move the vehicle, and to the regeneration operating mode, in which the electric machine operates as a generator to convert the kinetic energy of the vehicle, when this latter is decelerating, into electric energy.
In order to avoid interruption in the transmission of the torque (the so-called “torque hole”) when shifting from the first speed (i.e. the low speed) to the second speed (i.e. the high speed) in a two-speed transmission for electric vehicles, it is known from DE4212324 the use of an overrunning clutch associated to the driven gearwheel of the gearing of first speed. More specifically, the two-speed transmission known from this prior art document comprises a primary shaft, a secondary shaft, a gearing of first speed comprising a first driving gearwheel carried by the primary shaft and a first driven gearwheel carried by the secondary shaft and permanently meshing with the first driving gearwheel, a gearing of second speed comprising a second driving gearwheel carried by the primary shaft and a second driven gearwheel carried by the secondary shaft and permanently meshing with the second driving gearwheel, an overrunning clutch associated to the first driven gearwheel, a first coupling device arranged to couple the second driven gearwheel for rotation with the secondary shaft, and a second coupling device arranged to couple the first driven gearwheel, to which the overrunning clutch is associated, for rotation directly with the secondary shaft, so as to allow the transmission of the torque via the gearing of first speed also in the direction from the secondary shaft to the primary shaft. According to such a known solution, both the coupling devices are of the normally open type, that is to say, at rest (no command actively applied to the device) they are in the open condition in which they do not couple the respective gearwheel for rotation with the respective shaft. Moreover, the two coupling devices are controlled by a single control mechanism which alternatively closes either of the coupling devices.